1. Field of the Invention
This invention relates to loading dock equipment and in particular to mechanically dock levelers using a power assist employing a hydraulic actuator to move the deck of the dock leveler. The invention also uses a linkage coupled to the hydraulic actuator to allow the leveler to operate in a manual mode of operation. This invention is also related to the hydraulic and electrical circuitry used to operate the dock leveler.
2. Prior Art
Dock levelers employ a variety of techniques to raise the deck from a stored horizontal position to an elevated position where the lip can extend and the deck then "walked down" into position. Pit-style dock levelers are manufactured in a wide range of models to meet various requirements of size, capacity and performance. They can be divided into a number of different types as a function of the mode of power. Mechanical dock levelers are the most common because of their simplicity. They are widely available and for reference purposes U.S. Pat. No. 3,997,932 illustrates one example of such a mechanical dock leveler.
This type of dock leveler employs different techniques of powering the device. Manual lift mechanical dock levelers are common and again, simple. However, operation requires the user bending over and lifting the chain to release the unit. Another common type is the walk-down mechanical dock leveler. This requires less effort since the device is biased upward and the weight of the user "walks down" the device. However, the hold down requires maintenance and adjustment is required to maintain accurate spring balance.
There is a tendency in the industry toward automation to avoid physical strain and enhance loading dock operations. Thus it would be preferable to have remote actuation by push button without involving dock personnel in the manipulation of the leveler by pulling chains and the like to initiate operation.
A second type is the hydraulic dock leveler which features ease of use but at higher complexity due to the hydraulics which are employed as the exclusive mode of power. A variant is the hydraulic dock leveler with mechanical lip extension. This device has moderate complexity when compared with a hydraulic dock leveler with hydraulic lip extension. The latter device provides better control of lip motion but at a higher degree of complexity compared with those having mechanical lip extension.
Contemporary issues dealing with ease of use of these industrial products indicate a trend to move away from manually operated dock levelers. However riot all applications can justify the complexity and maintenance requirements of a hydraulic dock leveler. Also, some customers such as food and other environmentally conscious industries prefer non-hydraulic products to eliminate potential problems from hydraulic fluid spills. Certain non-standard fluids can be employed but require re-engineering and customization.
One device being offered to meet these demands replaces the hydraulic system with a blower and inflatable air bag to raise the dock leveler. This is illustrated in one form in U.S. Pat. No. 5,042,103. While this design provides a solution to some of the needs in the industry, it has unique operating characteristics. First, the air bag occupies most of the pit, making visual inspection and cleaning of the pit more time consuming. It will be appreciated that debris and the like frequently collect around the pit area and this area needs to be maintained clear of debris. Secondly, the air bag lift system does not have a capability to hold the leveler in a raised position for an "Emergency Stop" condition. Third, this type of leveler requires two people to install a support post under the leveler for maintenance, one to press the "Raise" button and one to place the support post under the deck.
Other configurations have been proposed using a large electric actuator to replace the hydraulic system, but large actuators, commercially available, are more expensive than hydraulic systems. Consequently, there exists in the dock loading industry a need for a simple yet powered assisted dock leveler.
In U.S. Pat. Nos. 5,826, 291 and 5,774,920 the assignee disclosed the use of an electric actuator as the source of power to assist in the operation of raising the deck as an alternative for these higher powered systems. In the '291 patent an electric actuator is to power assist a "manual lift" dock leveler. Since the least expensive type of mechanical pit-style dock leveler is this type the use of a power assist is a logical extension of the technology. Mechanical springs are used to partially offset the weight of the deck but the deck is still downward biased. The springs are used to reduce the force required by the electric actuator since the cost of actuators with lifting capacities over 1000 pounds would not provide an advantage over conventional hydraulic dock levelers. Also, the spring counterbalance enables the deck to be lifted by hand for maintenance or when electrical power is lost. The electric actuator provides a clean source of power and leaves the dock leveler pit open for easy inspection and cleaning.
Unlike a hydraulic cylinder which can "float" in response to external force, an electric actuator of the '291 patent cannot be forced follow the up and down motion of a dock leveler with the lip resting on a trailer. The use of an electric actuator conventionally requires separate control buttons to extend and retract the mechanism.
The use of an electric actuator also raises problems of alignment of the actuator and electrical wiring and cost considerations. If the source of power was interrupted the unit would not function. Moreover, when the leveler is restored to the cross-traffic position but is not fully restored, vibration and shock loads attendant with fork lift truck operation on the deck the electrical actuator may damage the unit. That is, the lip may not be fully seated in the lip keepers to support the deck so that the load will be carried by the electric actuator mechanical structure. In this circumstance the electric actuator will be subjected to the forces caused by movement of fork lift vehicles and the like on the deck until the unit settled into a horizontal position by repeated loads. Those forces would be beyond the rated capacity of the actuator and could damage the unit.
Consequently the need exists for a device offering the power assist of the '920 technology that is robust for the environment of the loading dock, yet reliable and cost effective.
It is an object of this invention to provide a mechanical dock leveler using simple hydraulic actuator technology to assist in raising the deck and requires less power for the actuator.
Yet another object of this invention is to define a simple actuator that employs a linkage member allowing the deck to be raised either hydraulically or manually and when manually operated will be self supporting in the absence of hydraulic power.
It is another object of this invention to define a method of control where the dock leveler utilizing simple controls with a minimum of switches.
Still another object of this invention is to define an operating system using standard 110 volt components.